Personally, I do it because it is easier and fits the general theme around the website, Ie some people have story's where Jon has an older sister or Sarah is nice or Zoe is a cheerleader or...and...then etc
And sure these are excellent deviations that help keep things interesting but in the general term of things Zoe is a goth, there's 3 Madison's or Gibson s, Sarah is a vindictive Cow and so on. And sure i get what you are saying Sarah is that characteristic from the point of view of Jon and Karyn, As long as I am thinking about a story from their perspective that is how she will be classified in my mindset.
Take the recent cheerleader Coven story, Jon has been forced against his will to be a cheerleader the squad, with the only name mentioned being Sarah. Some people might see the reason to cast her as the baddie in this scenario, sure enough it would be just as easy to write a few chapters Along the lines of Sarah did it for Jon's better good, Make her the nice girl, Jon Was "characteristically" a nobody and Sarah could have made it so Jon could be a Somebody, Nice Sarah, deserving of the head cheerleader status and stuff. There's also the fact that now Jasmine is a cheerleader she see's Sarah in a new light, Some confrontation could be added to make Sarah aware she seem cold in some situations And spin off to a whole new "better" Lake Point.
But then again Depending on the authors point of view they could Find more inspiration and more Content on following the Vindictive cow Scenario, This is just my take on one branch never mind the whole website, It is possible to Make a great story line with the Characters outside of there generalized characteristics but its also just as easy to make a potentially great story using previously crafted templates....I kind of lost my train of thought and ended this early...
